1. Field of the Invention
This invention relates to printed wiring boards and more particularly to printed wiring boards utilizing metal substrates.
2. Description of the Prior Art
In both additive and subtractive techniques of printed circuit manufacture, a great variety of base materials have been employed as an insulating support. In addition to thick metal blanks, e.g., 16 to 32 mils in thickness, there is great interest in using thin metal blanks, e.g., 1 to 7 mils in thickness, which are coated with dielectric material, for flexible printed wiring boards. In the past, thicker, non-flexible metal blanks, e.g., 16 to 32 mils in thickness, have been coated with dielectric materials using a pre-heated substrate and a fluidized bed powder coating process or an electrostatic coating process. Both of these techniques have a disadvantage where thin (1 to 7 mils thick), flexible metal blanks are contemplated which contain through holes. If such a thin metal blank is coated using known techniques, the dielectric coat obtained either does not provide adequate through hole edge coverage, or, if it does provide adequate edge coverage, the resultant coating is too thick or the surface topography thereof it too rough and not usable for practical purposes. If poor dielectric edge coverage is obtained, then a short will likely take place in the resultant printed wiring board between the metal blank or substrate and the conductive circuit pattern formed on the dielectric coat. If the dielectric coat is too thick, the flexibility of the resultant circuit suffers as well as results in an increase in material costs. Also, where the topography of the surface is rough and uneven, it is very difficult to print or stencil either a conductive pattern or a resist pattern thereon. Also, upon metallization of such a rough surface, the metal deposit obtained will have inherent mechanical stress therein resulting from rough topography.
A process which can yield both rigid and flexible metal printed wiring boards having a dielectric coated surface having good edge coverage as well as desirable topography properties is needed and is desired.